1. Field of the Invention
In general, the invention is directed to a method of separating solid particles of nearly uniform size but differing densities in a fluid medium. More particularly, the invention is directed to a method and apparatus for separating a mixture of cation and anion exchange resins into constituent types of resins so that the resins may be chemically regenerated.
2. Description of the Prior Art
Contaminated fluids are often treated with ion exchange resins to remove the contamination from the fluid. One particular application of ion exchange resins for cleaning such fluids is in the operation of electric power plants. The water used in all power plants gradually builds up contamination consisting of corrosion and erosion products, and impurities in the incoming water.
In order to prevent excessive build-up of the contamination of the water in a power plant with resulting fouling of components it has become standard operating practice both in fossil and nuclear plants to use ion exchange resins to polish the condensate from the steam turbine. Furthermore, ion exchange resins are frequently employed in other areas of the plant to produce make-up water, to purify the primary coolant by sidestream purification, to purify liquid waste effluents and, in the case of a nuclear reactor, to purify water in fuel storage pools. These ion exchange resins are contained in demineralizers or ion exchange resin beds that are normally comprised of two parts by volume of a cation resin and one part per volume of an anion resin. Although it is possible to employ cation and anion resins in separate resin beds, mixed resin beds are preferred because they have a greater ion exchange efficiency.
At a certain point in the operation of these resin beds, the resins used to remove contamination from fluids experience (1) an excessive build-up of contamination on the surface of the resin particles and (2) a depletion of the ion exchange capability of the resin particles. Because of the expense of ion exchange resins it is impractical to dispose of these resins after one cycle, and according to the current practice depleted ion exchange resins are rejuvenated by ultrasonic cleaning to remove suspended solids from the external surface of the resins and by chemical regeneration to restore the ion exchange capability of the resins.
The present art of chemically regenerating ion exchange resins consists of a chemical treatment and a washing cycle for each type of resin. After the mixed resin is separated into a cation fraction and an anion fraction the cation resin is rejuvenated by addition of dilute acid solution and the anion resin is rejuvenated by addition of a dilute caustic solution. The residual acid and caustic solutions are removed from the resins by thorough rinsing with high purity water before the resins are recombined by air mixing and returned to the demineralizer system. One of the most important steps in the chemical regeneration process is the separation step since, if an incomplete separation is made, when the chemical treatments are applied to the respective resin types the fraction of the other type of resin contained therein will be damaged and useless for further ion exchange purposes.
Therefore the first step in a chemical regeneration process is isolation or separation of the different types of resins. In the separation technique most commonly practiced in the prior art the resin bed is removed from the demineralizer tank and transferred to a tank equipped for chemical regeneration. In this first tank the resin bed is backwashed with water to expand the bed to about twice its original volume. The backwash water is then stopped and the resins are allowed to settle. Since the cation resins are more dense than the anion resins they settle first to the bottom of the tank and stratification of the cation and anion resins occurs. After the resins have completely settled the anion resins are removed from the tank by means of an outlet pipe fixed to the side of the tank where the bottom of the anion layer should be. An example of an ion exchange rejuvenation process employing this prior art technique of separating cation and anion exchange resins is found in U.S. Pat. No. 3,385,787 to Crits et al. The difficulties with this prior art technique of separating resins are that only a predetermined volume of mixed resin having predetermined fractions of cation and anion resins may be processed, and the layers of cation and anion resins seldom form an interface at the exact level of the outlet pipe. Thus, either some cation resins are withdrawn from the tank with the anion resins or some anion resins are left behind in the tank with the cation resins. Additionally, the interface formed between the separated anion and cation resins by this technique is often not clearly defined. In any case an incomplete separation is usually made by the current separation technique and when the chemical treatments are applied to the respective resin types the fraction that is of the other type is damaged and rendered useless for further ion exchange purposes.
Prior art attempts to improve cation and anion separation techniques involve the use of sight glasses or transparent tanks to monitor the position of the interface between the separated cation and anion exchange resins. Examples of these types of prior art devices are found in U.S. Pat. No. 3,429,807 to Burgess and U.S. Pat. No. 3,634,229 to Stanley, Jr. A problem with prior art separation techniques using a sight glass or the like is that a human operator is required and the process is not easily automated.
Another problem with prior methods of separating mixtures of anion and cation exchange resins is that prior art techniques employ a batch separation method which is time-consuming when large quantities of resin need to be separated and treated. Also, prior art batch separation techniques are often not capable of separating varying volumes of a mixture of ion exchange resins comprised of varying fractions of a cation and anion exchange resin. Prior art batch separation techniques are usually limited to a predetermined volume of a mixture of resins made up of predetermined fractions of cation and anion resins.
Therefore, it is an object of the present invention to provide a method and apparatus for precisely separating a mixture of cation and anion resins into its constituent parts for chemical regeneration with a minimum of waste.
It is another object of the present invention to provide a method and apparatus for separating various volumes of a mixture of cation and anion exchange resins.
It is another object of the present invention to provide a method and apparatus for separating a mixture of ion exchange resins made up of varying fractions of a cation and anion exchange resin.
It is another object of the present invention to provide an automated apparatus for separating the mixture of cation and anion resins that requires little or no attention by a human operator.
It is another object of the present invention to provide a method and apparatus for continuously separating a mixture of cation and anion resins which is quicker and more precise than prior art batch separation methods.